The effect of Theophylline on the induction of cytotoxic thymus derived lymphocytes (CTLs) in the in vitro allograft reaction is under analysis. Many diseases characterized by altered immunity are associated with abnormalities of suppressor cells. The rejection of foreign cells is mediated by CTLs, normally under the influence of suppressor cells. Theophylline can induce suppressor cell function. Theophylline-induced suppression is non-specific in that it occurs in the absence of antigenic stimulation; it is dose dependent; it is independent of the continued presence of the drug. Current studies are underway to define the cell lineage of this suppressor cell and to characterize its mechanism of action. Theophylline may prove a useful agent to probe the physiologic role of suppressor cells in immunologic disorders and for developing therapeutic strategies in disease states.